Several pen designs have been proposed wherein a valve plunger is operated to supply a writing pen with ink from an ink reservoir, the pen being used to write thick letters with ink which is of relatively high viscosity and has pigment particles dispersed in a solvent. A specific example of such a writing pen is described in U.S. Pat. No. 3,640,631. The primary feature of this prior art pen is the construction of a valve plunger which is used with a highly porous writing nib which easily absorbs ink through both the rear end and side surfaces. This nib may, for example, be a nib of relatively large diameter that is made of fairly loosely bound, resin-treated fiber filaments, or a felt material. But this arrangement cannot be directly used with a durable writing nib having an outer diameter of not more than 3 mm that is suitable for writing fine letters. Such a durable writing nib may, for example, be a synthetic resin nib having an ink passage bored through the axis of the interior of the resin, or a fiber nib comprising a firm combination of longitudinally bound filaments. A nib of this kind has a small capacity for retaining ink and absorbs only a limited amount of ink through its side surface. If the arrangement of U.S. Pat. No. 3,640,631 is directly applied to a fine nib, the small ink retaining capacity of the nib, and the direct supply of ink from an ink reservoir without control by capillary action, cause excessive ink to spill from the pen point.